The phospholipase A.sub.2 enzymes comprise a widely distributed family of enzymes which catalyze the hydrolysis of the acyl ester bond of glycerophospholipids at the sn-2 position. One kind of phospholipase A.sub.2 enzymes, secreted phospholipase A.sub.2 or sPLA.sub.2, are involved in a number of biological functions, including phospholipid digestion, the toxic activities of numerous venoms, and potential antibacterial activities. A second kind of phospholipase A.sub.2 enzymes, the intracellular phospholipase A.sub.2 enzymes, also known as cytosolic phospholipase A.sub.2 or cPLA.sub.2, are active in membrane phospholipid turnover and in regulation of inflammation mediated by the multiple components of the well-known arachidonic acid cascade. One or more cPLA.sub.2 enzymes are believed to be responsible for the rate limiting step in the arachidonic acid cascade, namely, release of arachidonic acid from membrane glycerophospholipids. The action of cPLA.sub.2 also initiates the biosynthesis of platelet activating factor (PAF). U.S. Pat. Nos. 5,322,776, 5,354,677, 5,527,698 and 5,593,878 disclose such enzymes (sometimes referred to herein as "cPLA.sub.2 .alpha.").
The phospholipase B enzymes are a family of enzymes which catalyze the hydrolysis of the acyl ester bond of glycerophospholipids at the sn-1 and sn-2 positions. The mechanism of hydrolysis is unclear but may consist of initial hydrolysis of the sn-2 fatty acid followed by rapid cleavage of the sn-1 substituent, i.e., functionally equivalent to the combination of phospholipase A.sub.2 and lysophospholipase (Saito et al., Methods of Enzymol., 1991, 197, 446; Gassama-Diagne et al., J. Biol. Chem., 1989, 264, 9470). Whether these two events occur at the same or two distinct active sites has not been resolved. It is also unknown if these enzymes have a preference for the removal of unsaturated fatty acids, in particular arachidonic acid, at the sn-2 position and accordingly contribute to the arachidonic acid cascade.
Upon release from the membrane, arachidonic acid may be metabolized via the cyclooxygenase pathway to produce the various prostaglandins and thromboxanes, or via the lipoxygenase pathway to produce the various leukotrienes and related compounds. The prostaglandins, leukotrienes and platelet activating factor are well known mediators of various inflammatory states, and numerous anti-inflammatory drugs have been developed which function by inhibiting one or more steps in the arachidonic acid cascade. The efficacy of the present anti-inflammatory drugs which act through inhibition of arachidonic acid cascade steps is limited by the existence of side effects which may be harmful to various individuals.
A very large industrial effort has been made to identify additional anti-inflammatory drugs which inhibit the arachidonic acid cascade. In general, this industrial effort has employed the secreted phospholipase A.sub.2 enzymes in inhibitor screening assays, for example, as disclosed in U.S. Pat. No. 4,917,826. However, because the secreted phospholipase A.sub.2 enzymes are extracellular proteins (i.e., not cytosolic) and do not selectively hydrolyze arachidonic acid, they are presently not widely believed to contribute to prostaglandin and leukotriene production. While some inhibitors of the small secreted phospholipase A.sub.2 enzymes have been reported to display anti-inflammatory activity, such as bromphenacyl bromide, mepacrine, and certain butyrophenones as disclosed in U.S. Pat. No. 4,239,780. The site of action of these compounds is unclear as these agents retain anti-inflammatory activity in mouse strains lacking sPLA.sub.2. It is presently believed that inhibitor screening assays should employ cytosolic phospholipase A.sub.2 enzymes which initiate the arachidonic acid cascade.
An improvement in the search for anti-inflammatory drugs which inhibit the arachidonic acid cascade was developed in commonly assigned U.S. Pat. No. 5,322,776, incorporated herein by reference. In that application, a cytosolic form of phospholipase A.sub.2 was identified, isolated, and cloned. Use of the cytosolic form of phospholipase A.sub.2 to screen for anti-inflammatory drugs provides a significant improvement in identifying inhibitors of the arachidonic acid cascade. The cytosolic phospholipase A.sub.2 disclosed in U.S. Pat. No. 5,322,776 is a 110 kD protein which depends on the presence of elevated levels of calcium inside the cell for its activity. The cPLA.sub.2 of U.S. Pat. No. 5,322,776 plays a pivotal role in the production of leukotrienes and prostaglandins initiated by the action of pro-inflammatory cytokines and calcium mobilizing agents. The cPLA.sub.2 of U.S. Pat. No. 5,322,776 is activated by phosphorylation on serine residues and increasing levels of intracellular calcium, resulting in translocation of the enzyme from the cytosol to the membrane where arachidonic acid is selectively hydrolyzed from membrane phospholipids.
In addition to the cPLA.sub.2 of U.S. Pat. No. 5,322,776, some cells contain calcium independent phospholipase A.sub.2 /B enzymes. For example, such enzymes have been identified in rat, rabbit, canine and human heart tissue (Gross, TCM, 1991, 2, 115; Zupan et al., J. Med. Chem., 1993, 36, 95; Hazen et al., J. Clin. Invest., 1993, 91, 2513; Lehman et al., J. Biol. Chem., 1993, 268, 20713; Zupan et al., J. Biol. Chem., 1992, 267, 8707; Hazen et al., J. Biol. Chem., 1991, 266, 14526; Loeb et al., J. Biol. Chem., 1986, 261, 10467; Wolf et al., J. Biol. Chem., 1985, 260, 7295; Hazen et al., Meth. Enzymol., 1991, 197, 400; Hazen et al., J. Biol. Chem., 1990, 265, 10622; Hazen et al., J. Biol. Chem., 1993, 268, 9892; Ford et al., J. Clin. Invest., 1991, 88, 331; Hazen et al., J. Biol. Chem., 1991, 266, 5629; Hazen et al., Circulation Res., 1992, 70, 486; Hazen et al., J. Biol. Chem., 1991, 266, 7227; Zupan et al., FEBS, 1991, 284, 27), as well as rat and human pancreatic islet cells (Ramanadham et al., Biochemistry, 1993, 32, 337; Gross et al., Biochemistry, 1993, 32, 327), in the macrophage-like cell line, P388D.sub.1 (Ulevitch et al., J. Biol. Chem., 1988, 263, 3079; Ackermann et al., J. Biol. Chem., 1994, 269, 9227; Ross et al., Arch. Biochem. Biophys., 1985, 238, 247; Ackermann et al., FASEB Journal, 1993, 7(7), 1237), in various rat tissue cytosols (Nijssen et al., Biochim. Biophys. Acta, 1986, 876, 611; Pierik et al., Biochim. Biophys. Acta, 1988, 962, 345; Aarsman et al., J. Biol. Chem., 1989, 264, 10008), bovine brain (Ueda et al., Biochem. Biophys, Res. Comm., 1993, 195, 1272; Hirashima et al., J. Neurochem., 1992, 59, 708), in yeast (Saccharomyces cerevisiae) mitochondria (Yost et al., Biochem. International, 1991, 24, 199), hamster heart cytosol (Cao et al., J. Biol. Chem., 1987, 262, 16027), rabbit lung microsomes (Angle et al., Biochim. Biophys. Acta, 1988, 962, 234) and guinea pig intestinal brush-border membrane (Gassama-Diagne et al., J. Biol. Chem., 1989, 264, 9470). U.S. Pat. Nos. 5,466,595, 5,554,511 and 5,589,170 also disclose calcium independent cPLA.sub.2 /B enzymes (sometimes referred to herein as "iPLA.sub.2 ").
An additional cPLA.sub.2, called "cPLA.sub.2 .beta.", has also been identified. cPLA.sub.2 .beta. is described in co-pending application Ser. No. 08/788,975. Similar to cPLA.sub.2 .alpha., cPLA.sub.2 .beta. is a calcium dependent phospholipase with preference for arachidonic acid at the sn-2 position of the phospholips. Significant homology of amino sequences is found between the alpha and beta isoforms in three regions, namely the CaLB domain, catalytic domains A and B (see FIG. 1). This finding leads to the notion that cPLA.sub.2 .alpha. is a member of a multi-gene family and that more than one intracellular phospholipase A.sub.2 might be involved in arachidonic acid release, mis regulation of which may lead to various human diseases including inflammation and cancer formation (Heasley et al., J. Biol. Chem. 1997, 272, 14501).
In addition, it is believed that the phospholipase enzymes may perform important functions in release of arachidonic acid in specific tissues which are characterized by unique membrane phospholipids, by generating lysophospholipid species which are deleterious to membrane integrity or by remodeling of unsaturated species of membrane phospholipids through deacylation/reacylation mechanisms. The activity of such a phospholipase may well be regulated by mechanisms that are different from that of the cPLA.sub.2 of U.S. Pat. No. 5,322,776. In addition the activity may be more predominant in certain inflamed tissues over others.
Therefore, it would be desirable to identify and isolate additional cPLA.sub.2 enzymes.